Communications is vital for corporate business. The telephone infrastructure of many enterprise telecommunication systems employs a PBX (private branch exchange). The PBX, in part, functions similar to a network switch by routing calls internal and external to the enterprise. PBX systems provide a mechanism for reducing the TCO (total cost of ownership) related to telephone communications by requiring only a small percentage of the total number of ports to support the many different employees. In an enterprise, where a large number of calls remain internal to the company, external PSTN (public switched telephone network) connectivity can be purchased at much lower concentration, for example, at a ratio of 1:10 assuming not more than ten percent of the calls will be routed to the PSTN.
If a call comes into the enterprise from the PSTN and is determined to be an unwanted call (e.g., one that is redirected outside the enterprise), it is forwarded back to PSTN, unnecessarily consuming enterprise two ports (incoming and outgoing). Should the number of redirected enterprise calls meet or exceed the limitations of the enterprise system, the enterprise call system becomes paralyzed in that no further calls (that may have value to the company) can be processed. As the number of outgoing calls from the enterprise to the PSTN increases, however, one remedy is to purchase additional trunks as the PSTN connectivity increases, further increasing TCO. Accordingly, it is important for the company to control the number of ports actually used for external calls to the PSTN in order to preserve the TCO.
Voice-over-IP (VoIP) technology is rapidly gaining in popularity as a telecommunications mechanism, not only for conducting telephone calls over data (or IP) networks, but also based on the available services such as caller ID, voice e-mail, call waiting, call forwarding, and so on. SIP (session initiation protocol) is an emerging standard for handling IP-based telephone calls, multimedia teleconferencing, and other types of realtime IP-based communications. SIP gateways can be employed as an interface between IP and telephony networks enabling IP-communications for endpoints, PBXs, and other systems.
Prior to SIP-based VoIP infrastructures, the PBX was made aware of every phone in the enterprise. This enabled the PBX to be knowledgeable about incoming calls from the PSTN that needed to be redirected back into PSTN. This knowledge enabled the PBX to trigger TBCT (Two B-Channel Transfer), a technology that addresses PBX incoming and outgoing call port problems by placing the switching back into the telephone company system. Excessive port consumption is common in conventional VoIP implementations or where a media server is employed for media termination to account for NAT (network address translation) or TDM (time division multiplex) to VoIP conversion. Legacy TDM networks resolved this issue by requesting bridging via TBCT which sends a Q.931 Facility message to bridge the calls on the PSTN carrier switch.
With the increasing adoption of SIP-based telecommunications in combination with PBX systems, and with the SIP server being external to the PBX, the PBX loses control of TBCT triggering, thereby increasing port usage by a factor of two for every call forwarded to the external PSTN.